This invention relates generally to navigation systems, and more particularly concerns improvements in navigation systems which use overlapping modulated beams to define a desired navigation path.
In certain navigation systems, such as an instrument landing system (ILS) for aircraft, two or more carrier beams of the same frequency are typically modulated with different audio frequencies and overlapped in transmission to produce a desired navigational path in space defined by a succession of spatial points at which the modulation signal levels from the overlapped beams are equal. In a typical ILS system, four orthogonal modulated beams are generated, a first pair of beams in one plane having a first carrier frequency providing glide slope (up-down) information, and a second pair of beams having a second carrier frequency and being mutually perpendicular to the first pair of beams in a second plane providing localizer (azimuth) information. The four modulated beams are generated by one or more transmitters positioned adjacent the aircraft landing runway and transmitted to an approaching aircraft. A receiver in the aircraft compares the signal level of the modulation signals in each of the first and second pairs of beams, respectively, and generates output signals representative of glide slope and localizer position of the aircraft relative to the desired landing path, which output signals are in turn applied to a visual indicator for inspection by the pilot. When the modulation signal levels of a given pair of beams are equal, the indicator needle will be centered, and the airplane is thus known to be on the desired landing path.
The receiver in the aircraft is typically linearly responsive to the received signals within the needle range of the indicator. Thus equal increments of increased distance or offset of the aircraft from the desired course will result in equal increments of needle movement in the indicator toward one or the other ends of the indicator scale. When the indicator needle is "pegged," (i.e. at one end of the scale), the pilot has no way of telling how much further, if any, his aircraft is offset from the desired course beyond the amount represented by the end of the scale. Thus, spatial boundaries for a navigation system are typically defined by the linear response range of the aircraft receiver, which is coordinated with the ends of the indicator scale. This is true for both glide slope and localizer signals.
For a typical ILS installation, the included angle of the localizer signal which can be incrementally sensed by the aircraft indicator is 3 to 31/2 degrees, while for glide slope it is approximately 3.degree., although the values may vary slightly from receiver to receiver. The included beam angle relative to the desired navigation path over which the indicating system is linearly responsive is referred to as the course width of the navigation system. The boundaries of the course width, between which the response of the aircraft receiver must be linear, are defined for aircraft instrument landing systems by the International Civil Aviation Organization (ICAO), which sets international standards for navigation aids, to which the FAA adheres, as 0.155 DDM (difference in depth of modulation), where DDM is defined as follows: